Most of the time, it works better to tile to the floor, since there will often be a few drips here and there and tile handles it better.

Tiling above the showerhead is a matter of preference. I tiled to the ceiling (and some actually tile the ceiling, too). Not because it is the right thing to do, but because I liked it in my situation. If your ceiling isn't level, it can be a pain. In my case, I was using granite tile, and made my own cove molding from 2" strips, with a back-bevel 45-degree. Since the tile was installed on the diagonal, it easily hides the height differences in my ceiling.